Recently, a next-generation wireless communication technology has been studied to achieve, for example, an increase in the speed and capacity of wireless communication in a wireless communication system such as a cellular system as one of mobile phone systems. For example, following formulation of a communication standard called “Long Term Evolution (LTE)”, the standards body of 3rd Generation Partnership Project (3GPP) has discussed about a communication standard called “LTE-Advanced (LTE-A)” to achieve further performance improvement on the basis of the wireless communication technology of LTE.
One of communication technologies that are likely to be introduced to LTE-A in the future and a basic technical discussion of which is currently carried out at 3GPP is direct communication between communication terminals called “device-to-device (D2D) communication”. In the conventional cellular communication, communication terminals close to each other communicate (i.e. transmit user data to one another) through a base station. In the D2D communication, however, the communication terminals close to each other directly communicate without routing communication signal through a base station. The D2D communication allows communication between the communication terminals even when the base station is not in operation.
In the D2D communication discussion, it is assumed that the D2D communication and cellular communication share the same wireless resource (radio frequency band) allocated for UL communication of the cellular communication. The discussion has been also made on introduction of a communication terminal capable of performing both of the cellular communication and the D2D communication. Thus, when the D2D communication is performed by using the uplink radio frequency band for cellular communication, the base station performs, in an identical radio frequency band, both of allocation of an uplink wireless resource for cellular communication and allocation of a wireless resource for D2D communication to one communication terminal.
In the current LTE specifications, it is stipulated that layer 1 control information transmitted from the base station to the communication terminal is called “downlink control information (DCI)” and employs any one of Formats 0, 1, 1A, 1B, 1C, 1D, 2, 2A, 2B, 2C, 2D, 3, 3A, and 4 depending on its usage, in other words, the content of control information. For example, Format 0 or 4 is employed for DCI used when the base station notifies the communication terminal of an allocation result of a wireless resource to be used by the communication terminal to transmit a signal to the base station.
DCI is transmitted from the base station to the communication terminal through a “physical downlink control channel (PDCCH)” as one of wireless physical channels used in the LTE system. Each PDCCH is mapped to a wireless resource region including one or a plurality of continuous control channel elements (CCEs). The PDCCH employs any of Formats 0 to 3 depending on its size. The PDCCH in Format 0 has a size of “N” corresponding to “1CCE”, and the PDCCH in Format 1 has a size of “2N” corresponding to “2CCE”. The PDCCH in Format 2 has a size of “4N” corresponding to “4CCE”, and the PDCCH in Format 3 has a size of “8N” corresponding to “8CCE”. Thus, the sizes N, 2N, 4N, and 8N of the PDCCH correspond to the number of coupled CCEs, 1, 2, 4, and 8, respectively, where the number of coupled CCEs is called an “aggregation level”.
DCI is encoded at a code rate in accordance with the quality of a downlink propagation channel, specifically, at a lower code rate as the quality of the downlink propagation channel decreases. Thus, the size of encoded DCI increases as the quality of the downlink propagation channel decreases. When the encoded DCI is transmitted through the PDCCH, the size of the encoded DCI is adjusted to match with any one of the four sizes N to 8N of the PDCCH through rate matching. Accordingly, the PDCCH having a larger size is used for DCI transmission as the quality of the downlink propagation channel decreases, and the aggregation level is selected from among 1, 2, 4, and 8 depending on the size of encoded DCI. A CCE modulation scheme is fixed by quadrature phase shift keying (QPSK) irrespective of the quality of the downlink propagation channel.
A wireless resource region to which a PDCCH for each communication terminal is mapped is called a “search space”. As illustrated in FIG. 1, the search space is defined for each aggregation level. FIG. 1 is a diagram used for description of conventional search spaces. In FIG. 1, “SS” indicates the search space, and “AL” indicates the aggregation level. In the current LTE, for cellular communication, six search spaces SS0 to SS5 are defined depending on the aggregation level as illustrated in FIG. 1. Among search spaces SS0 to SS5, four search spaces SS0 to SS3 are unique to each communication terminal, and two search spaces SS4 to SS5 are common to all communication terminals.
In FIG. 1, SS0 at AL=1 includes six search units to which the PDCCH in Format 0 can be mapped, each search unit corresponding to 1CCE. SS1 at AL=2 includes six search units to which the PDCCH in Format 1 can be mapped, each search unit corresponding to 2CCE. SS2 at AL=4 includes two search units to which the PDCCH in Format 2 can be mapped, each search unit corresponding to 4CCE. SS3 at AL=8 includes two search units to which the PDCCH in Format 3 can be mapped, each search unit corresponding to 8CCE. SS4 at AL=4 includes four search units to which the PDCCH in Format 2 can be mapped, each search unit corresponding to 4CCE. SS5 at AL=8 includes two search units to which the PDCCH in Format 3 can be mapped, each search unit corresponding to 8CCE.
A 16-bit cyclic redundancy check (CRC) bit masked with a 16-bit string indicating the ID of a communication terminal at the destination of DCI is added to the DCI before encoding so as to identify the communication terminal. The communication terminal performs CRC by demasking a CRC bit part of a decoded bit string with the ID of the communication terminal, thereby detecting the DCI addressed to the communication terminal. In other words, the communication terminal determines that received DCI is the DCI addressed to the communication terminal if the CRC by demasking with the ID of the communication terminal is successful. Such detection of DCI by CRC using the ID of the communication terminal is also called “blind detection”.
One subframe includes SS0 to SS3 for each communication terminal, and SS4 and SS5. The communication terminal performs blind detection for each search unit included in each search space. As illustrated in FIG. 1, the total number of search units in SS0 to SS5 is 22. The size of DCI before encoding varies with each format, and DCI has a size of two kinds. Thus, the communication terminal performs blind detection for each of DCI having a size of two kinds for each search unit. Accordingly, the number of times of blind detection performed in one subframe is 22×2=44 at maximum for each communication terminal.
Examples of related-art are described in: 3GPP TR 36.913, “Requirements for further advancements for Evolved Universal Terrestrial Radio Access (E-UTRA) (LTE-Advanced)”, V9.0.0, Release 9, December 2009; 3GPP TR36.912, “Feasibility study for further advancements for E-UTRA (LTE-Advanced)”, V9.3.0, Release 9, June 2010; 3GPP TS36.321, “Medium Access Control (MAC) protocol specification”, V10.2.0, Release 10, June 2011; 3GPP TS36.133, “Requirements for support of radio resource management”, V10.3.0, Release 10, June 2011; 3GPP TS36.213, “Physical layer procedures”, V10.2.0, Release 10, June 2011; and 3GPP TS36.300, “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN)”, V10.4.0, Release 10, June 2011.
The current LTE defines no format for DCI used by a base station to notify a communication terminal of an allocation result of a D2D communication wireless resource. Thus, DCI in a new format can be introduced to notify an allocation result of a D2D communication wireless resource. However, the introduction of D2D communication DCI in a new format leads to an increase in the number of times of blind detection at the communication terminal.
FIG. 2 is a diagram used for description of a problem. In FIG. 2, “SS” indicates a search space, “AL” indicates an aggregation level, and a number in parentheses indicates the number of search units included in each search space. As described with reference to FIG. 1, the number of search units for cellular communication is 22 for each communication terminal. Conventionally, DCI has a size of two kinds, and thus the number of times of blind detection for cellular communication is 44 at maximum in one subframe for each communication terminal as described above. However, for example, when SS6 to SS9 are prepared for D2D communication DCI in a new format similarly to the conventional SS0 to SS3 prepared for each communication terminal, the number of times of blind detection for D2D communication is 6+6+2+2=16 at maximum in one subframe for each communication terminal. Accordingly, blind detection is performed 44+16=60 times at maximum in one subframe at a communication terminal capable of performing both of cellular communication and D2D communication. In other words, a processing amount for the blind detection increases by 36% approximately from the conventional processing amount. The increase in the number of times of blind detection results in an increase in electric power consumption of the communication terminal, and thus it is preferable to decrease the number of times of blind detection.
The technology disclosed in the present application is intended to solve the above-described problem, and it is an object of the disclosed technology to reduce an increase in electric power consumption at a communication terminal in D2D communication.